Present day treatment for acute myocardial infarction entails intravenous administration of a thrombolytic drug within the first few hours after onset of symptoms to break up (lyse) blood clots within coronary arteries thereby reversing damage to the affected heart muscle. There are two major thrombolytic drugs now available, streptokinase and tissue plasminogen activator (TPA).
The activation of plasminogen by either streptokinase or TPA results in the formation of plasmin, a proteolytic enzyme that degrades fibrin, the principal component of the lattice which holds a blood clot together.
Streptokinase is a naturally occurring product from the bacteria streptococci. Because streptokinase is a bacterial product and an antigen, many individuals who have had previous streptococcal infections (e.g., strop throats) have anti-streptokinase antibodies in their blood. These antibodies neutralize streptokinase when it is administered as a drug (Brogden, R. N., et al., Drugs: 5:357-445 (1973)). Anti-streptokinase titers between 2 to 402 U/ml in a random sample of 120 people has been reported (Bachmann, F., J. Lab. Clin. Med. 72:228 (1968)). Streptokinase is biochemically inert when bound to this antibody and the complex of streptokinase and antibody is rapidly cleared from the circulation (Fletcher, A. P. et al., Clin. Invest. 37:1306 (1958)). Such antistreptokinase antibodies may account for some treatment failures of streptokinase in myocardial infarction due to inadequate dosing with the drug.
It is necessary to begin thrombolytic therapy early after onset of myocardial infarction (within 4 hours) to achieve satisfactory clinical results. Consequently, the choice of which drug to use and how much to use should be made quickly. For effective therapy when streptokinase is chosen, the dose of streptokinase must begin with a dosage in excess of that required to neutralize endogenous circulating antibodies to streptokinase (Bfogden, R. N., et al,, Drugs: 5:357-445 (1973)). Doses which are not in excess of the amount required to neutralize these endogenous antibodies are pharmacologically inactive. It is critical to determine the proper dose for streptokinase therapy. Doses which are too high may lead to the formation of excessive plasmin and result in the depletion of additional proteins which plasmin also degrades, such as circulating fibrinogen and clotting factors V and VIII. Thus, with too much streptokinase there is a risk of hemorrhage. When the hemorrhage occurs in the central nervous system, grave neurological impairment or death usually results (Braunwald, E. et al., J. Am. Coll. Cardiol. 10:970 (1987); Haber, E. et al., Science 243:51 (1989)).
Because there is such individual variability in the levels of antistreptokinase antibodies, a rapid determination of the presence of significant amounts of anti-streptokinase antibodies in a patient would be very useful in guiding medical decisions concerning the dosage of streptokinase needed for therapeutic treatment. With the knowledge that antistreptokinase antibodies are present, an appropriate initial neutralizing dose may be administered, followed by an infusion of the drug in an amount sufficient to maintain the level of free streptokinase required for the induction of a thrombolytic state.
Such an assay would also be useful as part of routine cardiac risk assessment profiling, especially, after a patient has been treated with streptokinase. Eight to nine days after treatment, the titer of antistreptokinase antibodies rapidly rises 50-fold to 100-fold in most patients, only returning to pretreatment levels 4-6 months later (Schmutzler, R. et al., Thrombolytic Therapy, in Poller Recent Advances in Blood Coagulation, p. 324, Churchill, London, 1969). Consequently, it is critical to know the titer of streptokinase antibodies in these patients before repeating a course of streptokinase within a few months of the original treatment.
Further, just as cholesterol is used as a screening test for risk of developing heart disease, it is desirable to routinely prescreen individuals for antistreptokinase antibody status on a regular basis. The results of such a prescreen would be of value for individuals who wish to know if streptokinase would be efficacious in treating their future heart-attacks.
Thus a need exists for a rapid, economical test for antistreptokinase antibodies.
It is known that the bacterial thrombolytic agent streptokinase binds to human, porcine, and chicken LD isoenzyme subunit M, but not to the H or C subunits (Podlasek, S. J. et al., Clin. Chem. 35:69-73 (1989)). There is amino acid sequence homology between LD and the streptokinase binding site on plasminogen to account for this interaction. The binding of streptokinase to LD subunit M results in the formation of a streptokinase-LD complex in serum that contain LD activity (Podlasek, S. J. et al., Clin. Chem. 35:69-73 (1989)). However, it has not previously been known to use the affinity of streptokinase for LD as the basis for the determination of antistreptokinase antibodies in the serum of an individual.